Synthesis of chemically reactive ceria composite nanoparticles and CMP applications thereof

ABSTRACT

The present invention provides a method of synthesizing nanosized abrasive particles and methods of using the same in chemical mechanical polishing slurry applications. The nanosized abrasive particles according to the invention are produced by hydrothermal synthesis. The crystallites of the particles include cerium atoms and atoms of metals other than cerium. In a preferred embodiment of the invention, the crystallites exhibit a cubic crystal lattice structure. The differences in electric potential between the cerium atoms and the atoms of metals other than cerium facilitate the polishing of films without the need for chemical oxidizers.

RELATED APPLICATIONS

This application is a continuation-in-part of co-pending Appn. Ser. No.10/255,136, filed Sep. 25, 2002, which is a continuation in part ofAppn. Ser. No. 09/992,485, filed Nov. 16, 2001, now U.S. Pat. No.6,596,042.

FIELD OF THE INVENTION

The present invention provides a process for producing abrasiveparticles, the abrasive particles produced according to the process, anda process for removing a film layer using a CMP slurry containingparticles made by the process.

BACKGROUND OF THE INVENTION

Chemical-mechanical polishing (CMP) slurries are used, for example, toplanarize surfaces during the fabrication of semiconductor chips andrelated electronic components. CMP slurries typically include reactivechemical agents and abrasive particles dispersed in a liquid carrier.The abrasive particles perform a grinding function when pressed againstthe surface being polished using a polishing pad, and separately, thereactive chemical agents serve to oxidize the surface.

It is well known that the size, composition, and morphology of theabrasive particles used in a CMP slurry can have a profound effect onthe polishing rate and surface finishing. Over the years, CMP slurrieshave been formulated using abrasive particles formed using, for example,alumina (Al₂O₃), cerium oxide, or ceria (CeO₂), iron oxide (Fe₂O₃),silica (SiO₂), silicon carbide (SiC), silicon nitride (Si₃N₄), tin oxide(SnO₂), titania (TiO₂), titanium carbide (TiC), tungsten oxide (WO₃),yttria (Y₂O₃), zirconia (ZrO₂), and combinations thereof.

Known abrasive particles for use in CMP slurries include colloidalsilica, which is produced by condensation in aqueous solution. Anotheris fumed silica, which may be produced by a continuous flame hydrolysistechnique involving the conversion of silicon tetrachloride (SiCl₄) tothe gas phase using an oxy-hydrogen flame. The silicon tetrachloridereacts with the combustion by-produce (water) to yield silica (SiO₂) andhydrochloric acid: SiCl₄+2H₂O→SiO₂+4HCl. The HCl is easily separated asit remains in the gas phase, while the fumed silica is solid. In orderto attain a desired particle size, the fumed silica is mechanicallyground or milled. Fumed silica is by far the most widely used abrasiveparticle.

Calcination is another method of producing abrasive particles for use inCMP slurries. During the calcination process, precursors such ascarbonates, oxalates, nitrates, and sulfates, are converted into theircorresponding oxides at very high temperature. After the calcinationprocess is complete, the resulting oxides must be milled to obtainparticle sizes and distributions that are proper to provide desiredremoval rate and prevent scratching.

The calcination process, although widely used, does present certaindisadvantages. For example, it tends to be energy intensive and canproduce toxic and/or corrosive gaseous byproducts. In addition,contaminants are easily introduced during the calcination and subsequentmilling processes. Finally, it is difficult to obtain a narrow particlesize distribution.

The basic mechanism of the CMP process is the simultaneous formation ofa removable surface layer, such as via oxidation of a metal surface orvia hydrolysis of an oxide or nitride layer, coupled with the mechanicalremoval of the removable surface layer using abrasive particles pressedbetween the work piece and a polishing polishing pad that are in motionrelative to each other. In CMP slurries for removing copper films, themechanical (abrasive) effect and oxidizing function are separatelyprovided by the different components. That is, abrasive particles mainlycontribute the mechanical effect, while chemical oxidizing agents giverise to a chemical (redox) reaction.

Numerous chemical additives exist to improve film removal rates, toadjust the selectivity of removal rates between various materials, andto allow better surface finishing and less defects. Hydrogen peroxide,ferric nitrate, potassium iodate and periodate are widely used asoxidizing chemicals in copper CMP slurries to improve removal ratesrelative to slurries having only abrasive particles. Most CMP slurriesare formed by combining two separate components, namely: (1) abrasiveparticles dispersed in a liquid medium; and (2) chemical additives(e.g., a chemical oxidizer). The separate components are mixed togetherimmediately prior to use and, once blended, have a shelf life oftypically only about 5 days or less. The chemical oxidizer inconventional CMP slurries tends to lose its oxidative efficacy if itremains unused for long periods.

While the use of chemical oxidizers improves the metal removal rate toindustrially practicable levels, the chemical oxidizers in the slurrycontinue to oxidize metal until they are expended or removed. Hence,chemical oxidizers are one of main contributors to the problem ofdishing or pitting of metal surfaces, which results from continuedoxidative attack on an already planar metal surface, even in the absenceof abrasive particles.

BRIEF SUMMARY OF THE INVENTION

Broadly, the abrasive particles according to the invention comprisecrystallites (primary particles) that include cations of cerium andcations of at least one other metal, which have been formed byhydrothermal synthesis. The abrasive particles can be used to formulateCMP slurries that provide industrially acceptable removal rates of avariety of surface films (substrates), without the need for addedchemical oxidizers, which eliminates concerns about dishing and cupping.Slurries formulated using abrasive particles according to the inventionexhibit a shelf life far greater than traditional CMP slurries. Anotheradvantage provided by the abrasive particles according to the inventionis that use of chemical oxidizers can be avoided, which reduces theenvironmental impact of producing electronic components.

The foregoing and other features of the invention are hereinafter morefully described and particularly pointed out in the claims, thefollowing description setting forth in detail certain illustrativeembodiments of the invention, these being indicative, however, of but afew of the various ways in which the principles of the present inventionmay be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a Table displaying several properties and polishing rates ofthe abrasive particles formulated in Example 5.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides a method of synthesizing abrasive particleshaving a desired reactivity, which can be used in formulating CMPslurries exhibiting a variety of substrate removal rates. The inventiveparticles may be used to polish metal substrates or metal oxidesubstrates. Both operations are routinely required in the manufacture ofelectronic components. The substrate removal rate of CMP slurries isbelieved to depend on numerous factors. A non-exhaustive list of suchfactors includes: the composition of the abrasive particles; therelative level and identity of guest metal ions in those particles; thesize of the primary particles (i.e., crystallite size); the size ofsecondary particles (i.e., coalesced or agglomerated primary particles);the concentration of abrasive particles in the slurry; the pH of theslurry; and the presence and concentration of chemical oxidizers in theslurry. The focus of the present invention is the control of primary andsecondary particle size of the abrasive particles, and the control ofthe level of guest ions in such primary particles. Chemically reactiveabrasive particles according to the invention can be used in the CMPprocess to produce both chemical and mechanical effects.

It has been discovered that nanoscale composite ceria particles can besynthesized hydrothermally such that cerium oxide acts as a host matrix(crystal lattice structure) for guest metal atoms (or ions) that takethe place of cerium atoms (or ions) in the crystal lattice structure ofthe host matrix. The inventive process produces “nanoscale” particles,with primary particles (crystallites) having a mean diameter (D₅₀) inthe range of about 1 nm to about 10000 nm. In a preferred embodiment,the average crystallite size may be about 5 to about 1000 nm, morepreferably about 10 to about 400 nm, still more preferably about 15 toabout 200 nm, and even more preferably about 20 to about 100 nm.Secondary particles, which are agglomerations of primary particles,exhibit sizes within the range of from about 10 to about 10000 nm, butare preferably 30 nm to about 1000 nm, more preferably from about 40 toabout 800 nm and still more preferably from about 50 to about 500 nm.Throughout the instant specification and in the appended claims, theterm “particle” when used without further explanation refers tosecondary particles.

Generally, guest metal ions are substituted for a cerium ions in thecrystal lattice structure, thus preserving the ratio of metal cations tooxygen ions in such crystal. Thus, for cerium oxide, the mole ratio ofmetal atoms (or cations) to oxygen atoms in the crystal latticestructure will be about 1:2, although mole ratios of metal atoms tooxygen atoms of about 1:1.5 to 1:3.5 are possible. The ratio of ceriumatoms to oxygen atoms in the crystal is sufficient to preserve overallstatistical electroneutrality. The resultant composite metal oxideformula is thus Ce_(x)M_(y)O_(z) where x+y is about 1 and z is withinthe range of from about 1.5 to about 3.5. Because the guest metal ions(1) may have a different oxidation state than cerium and/or (2) do havea different electronegativity than cerium, the difference in electricalpotential is generally sufficient to drive a redox reaction on thesurface of a film to be polished when the abrasive particles accordingto the invention are in contact therewith. Films or substrates that canbe polished (removed) using abrasive particles according to theinvention include metals, metal oxides, metal nitrides, silicides, andpolymers.

In particular, the invention provides a method of producing abrasiveparticles for use in CMP slurries comprising providing an aqueousreaction mixture comprising a source of cerium ions and a source ofmetal ions other than cerium. The metal ions other than cerium areselected from the group consisting of Be, B, Mg, Al, Si, Ca, Sc, Ti, V,Cr, Mn, Fe, Co, Ni, Zn, Ga, Ge, As, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd,Ag, Cd, In, Sn, Sb, Te, Ba, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Hp, Er,Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi andcombinations thereof. The aqueous reaction mixture is subjected tohydrothermal treatment at a temperature of from about 70° C. to about500° C. to produce the abrasive particles. When the mole percentage ofcerium atoms in the crystal lattice structure of the crystallitesexceeds the mole percentage of guest metal ions, the crystallites willgenerally exhibit a cubic crystal lattice structure wherein ceriumserves as a host cation and the metal other than cerium serves as aguest cation. When the mole percentage of cerium atoms in the crystallattice structure is less than the mole percentage of guest metal ions,the crystallites will generally exhibit a crystal lattice structuredetermined by the guest metal ions, with the cerium cation serving asguests therein. The inventive abrasive particles can be dispersed inwater to form CMP slurries. If desired, a pH adjuster may be added toadjust the pH to about 3 to about 11.

The precise stoichiometry used in hydrothermal synthesis dictates thefinal aggregate ratio of guest to host ions in the crystal structures ofa sample of the inventive composite particles. An analysis of an aqueousdispersion of composite abrasive particle formed in accordance with theinvention that included cerium as a host cation and titanium as a guestion showed that the dispersion consisted mainly of small particleshaving a particle size distribution in the range of 6 to 30 nm, alongwith a few larger particles having a size of about 160 nm. The smallerparticles exhibited a spherical shape, whereas the larger particlesappeared to be more rectangular. The EDS spectrum for the largeparticles showed that they consist of 95 wt % ceria and 5 wt % titania,while the smaller particles consist of 58 wt % ceria and 42 wt %titania. Thus, it will be appreciated that variations in the relativemole percentage of cerium ions to guest ions will occur in individualparticles, which may be different than the average relative molepercentage for the bulk of the particles. In other words, while theaggregate composition of a dispersion of inventive particles will have amakeup approximately the same as the relative amounts of reagents used,the composition of a single given particle, or any subset of particlesfrom such a dispersion may have a composition widely different from theaggregate.

In the aggregate, the mole ratio of cerium ions to guest ions inabrasive particles according to the invention will be from about100000:1 to about 1:100000. In order to retain the desired cubic crystallattice structure of cerium oxide, the mole ratio of cerium ions toguest ions in the abrasive particles according to the invention will bewithin the range of from about 1000:1 to about 1:1. Still morepreferably, the aggregate mole ratio will be within the range of fromabout 90:1 to about 1.5:1. It will be appreciated that as theconcentration of guest ions increases, additional crystalline phasestend to appear in the primary particles. For example, crystallitescontaining substantial molar quantities of ceria and titania willexhibit both a cubic crystalline phase and an anatase crystalline phase,although both crystals contain ions of both cerium and titanium.

The cerium host ions may be provided by a Ce(III) salt or a Ce(IV) salt,for example Ce(NH₄)₂(NO₃)₆. The hydrothermal treatment may be carriedout for about 10 minutes to about 48 hours, preferably about 15 minutesto about 24 hours and more preferably about 1 hour to 8 hours. It isbelieved that the method of mixing the aqueous reaction mixture and abase has an effect on primary and secondary particle sizes. For example,blending a base into the aqueous reaction mixture by double jetinjection is effective in synthesizing composite ceria particles havingdesired small crystallite sizes. Ceria particles having crystallitesizes of about 10 nm to about 100 nm provide superior polishing rates inCMP operations.

The reaction mixture may be heated to a temperature of about 70° C. toabout 500° C., or to a temperature of about 200° C. to about 400° C.Preferably the second reaction mixture is heated to a temperature ofabout 300° C.

The source of guest ions is not critical. Accordingly, the source ofguest metal ions may be a salt of the guest ion. The guest ion salt maybe selected from the group consisting of nitrates, chlorides,perchlorides, bromides, sulfates, phosphates, carbonates, and acetatesof the guest ion. Metal ethoxides may also be used. In a preferredembodiment, the source of guest ions may be Fe(NO₃)₃, Cu(NO₃)₂,Nd(NO₃)₃, or the hydrated forms thereof.

These composite particles can be used as chemically reactive abrasivesin CMP slurries to remove metal film layers such as copper without theneed for added oxidizers. Metal oxides, nitrides, silicides and polymersmay also be polished effectively. Metals that can be polished by theinventive methods include silver, gold, platinum, copper, palladium,nickel, cobalt, iron, ruthenium, iridium, and osmium, silicon, aluminum,germanium, tungsten, tantalum and alloys or blends thereof.

Metal oxides that can be polished by the inventive methods includeoxides of metals such as boron, sodium, magnesium, aluminum, silicon,phosphorus, potassium, calcium, gallium, germanium, arsenic, selenium,rubidium, strontium, yttrium, zirconium, tin, antimony, cesium, andbarium. Metal oxide substrates may also contain more than one of theaforementioned oxides. Metal nitrides can also be polished.

Low K-value dielectric materials may also be polished. Many of these arepolymeric, for example poly-para-xylenes commercially available from S.C. Cookson, of Indianapolis, Ind., under the Parylene tradename. Furthersuch polymers include fluorinated polyimides, methyl silsesqioxane, andpoly-(arylene ether)s. The Dow Chemical Company, of Midland, Mich.,commercially supplies B-staged polymers including those sold by underthe Cyclotene® and SiLK® trademarks. For example, Cyclotene® 4026-46 isa blend of B-staged divinylsiloxane-bis-benzocyclobutene, mesitylene,polymerized 1,2-dihydro-2,2,4-trimethylquinoline,2,6-bis{(4-azidophenyl) methylene}-4-ethylcyclohexanone, and1-1′-(1-methylethylidene) bis {4-(4-azidophenoxy)benzene}. In general,the Cyclotene® dielectric polymers contain at least B-stageddivinylsiloxane-bis-benzocyclobutene and mesitylene. Polymers sold underthe SiLK® trademark are semiconductor dielectric resins comprise a Dowproprietary b-staged polymer, cyclohexanone, and gamma-butyrolactone. Inaddition, carbon doped silica substrates, which are also known as SiCOHsubstrates, can be polished.

The difference in valence state and/or electronegativity between ceriaand the guest ions in the abrasive particles according to the inventiongives the particles the ability to provide the redox potential to filmsto be removed. Hence, it is believed that such oxidation occurs onlywhen a particle contacts the substrate surface, and consequently, thatoxidation and mechanical abrasion occur simultaneously. This fact,combined with the small particle size disclosed herein (nanoscale)provides extremely precise polishing, and planarization that is bothlocally and globally accurate. Thus, the invention further provides amethod of removing a film by CMP at a desired rate in the absence ofchemical oxidizers.

When all other polishing conditions are kept similar, adjustments in therelative molar percentage of guest metal ions in the crystal latticestructure of the abrasive particles according to the invention can beused to determine or tune the rate of film removal. Thus, the inventionfacilitates determining the rate of removal of a film layer by selectingthe composition of the abrasive, rather than adjusting other polishingparameters.

The invention further provides a method of removing a portion of asubstrate by contacting the substrate with a CMP slurry comprisingcomposite abrasive particles wherein the abrasive particles comprisecerium ions and guest metal ions selected from the group consisting ofiron, copper, neodymium, and combinations thereof, and wherein theslurry contains no additional oxidizers.

The invention also provides a method of producing a CMP slurrycomprising contacting the particles discussed hereinabove with water toform a suspension and adjusting the pH of the suspension to about 2.0 toabout 11.0 with a pH adjuster, wherein the slurry is substantially freeof chemical additives and oxidizers.

Because the particles of the present invention are so small, i.e.,primary particle diameters on the order of nanometers, a very highfraction of the atoms reside at the particle crystalline surfaces andgrain boundaries. As primary particle size decreases, the BET specificsurface area (m²/gram) increases. Without being limited to any theory,applicants postulate that nanoparticles are much more reactive than thecorresponding bulk material due to vastly increased specific surfacearea. It is further believed that surface defects, non-balanced charges,guest ions in the grain boundary and vacancies in the crystallinelattice and other surface active sites may beneficially induce orcatalyze chemical redox reactions.

Hydrothermal synthesis of cerium oxide abrasive particles is disclosedin commonly owned U.S. Pat. No. 6,596,042, which is hereby incorporatedby reference. In order for the particles to function as chemicallyactive nanoparticles, the nanoparticles must be able to form a stablesuspension in water. In developing the embodiments of the presentinvention, hydrothermal synthesis of ceria was carried out in order tofacilitate crystallization of the desired abrasive oxide particlescontaining guest cations.

With respect to the present invention, hydrothermal synthesis isconducted in a sealed (i.e., air-tight) container, typically made ofstainless steel. A metal salt containing the host cation ingredient,i.e., cerium ions, is solubilized in deionized water. A crystallizationpromoter is added, and optionally, a stabilizer for the crystallizationpromoter is also added. The pH of the inherently acidic salt solution israised to at least 1.5, preferably to at least 7.5, and more preferablyto at least 9.0 using a base, (which may be provided in the form of asolution), which assists in the formation of a solution having agel-like consistency. Suitable bases include, for example, NaOH, KOH,NH₄OH, organoamines such as urea, ethylamine and ethanolamine, and/orpolyorganoamines such as polyethylenimine. Combinations of bases canalso be used. Other compounds such as urea can also be added to assistin crystal growth. The gel-like solution will break down into smallparticles upon rapid stirring. Double injection mixing can be used toensure stoichiometric homogenization of the metal salt solutions withthe base(s) to ensure uniform crystal seed generation. Hydrothermalsynthesis is carried out in a closed container because the pressuregenerated by the raised temperature results in small particles having anarrow size distribution resulting from uniform crystal growth. Thesolution may be further diluted with deionized water as needed. Thesolution is transferred to a sealed stainless steel reaction vessel,which is heated to 70-500° C. for about 1 hour to about 500 hours understirring. The vessel is then cooled to room temperature, and the slurryis washed until a low conductivity is achieved, typically <5 mS,preferably <1 mS, more preferably <0.5 mS and most preferably <0.1 mS.As a final purifying step, the particles may be filtered using micronrange filter paper.

Crystallization promoters include titanium chloride, titanium sulfate,titanium bromide, organotitanium compounds such as titanium oxychlorideand those sold by E.I. DuPont de Nemours of Wilmington, Del., under theTyzor® trademark, for example Tyzor®-TE. A preferred crystallizationpromoter is Ti[OCH(CH₃)₂)]₄ (titanium (IV) isopropoxide). It will beappreciated that certain titanium compounds tend to rapidly decompose inaqueous media, which reduces their efficiency in promoting the formationof particles having larger crystallite sizes. Accordingly, it ispreferable for one or more stabilizing compounds such as, for example,acetyl acetone, (i.e., acetyl acetone titanate) to be present with thecrystallization promoters in order to prevent or delay theirdecomposition. Further details on crystallization promoters andstabilizers can be found in commonly owned U.S. Pat. No. 6,596,042, andcopending application Ser. No. 09/992,485. The guest ions areincorporated into the cerium oxide crystal structure without disruptingthe cubic structure thereof.

Although the preceding description and following experimental exampleswill show that the inventive abrasive particles alone—absent oxidizersand other chemical additives—can be used to exhibit a wide variety ofsatisfactory metal removal rates on a copper substrate, the metalremoval rates of any inventive particle can be further adjusted by theuse of chemical additives, including oxidizers. Such chemical additivesinclude hydrogen peroxide, ascorbic acid, citric acid, formic acid,acetic acid, propionic acid, butyric acid, valeric acid, acrylic acid,lactic acid, succinic acid, nicotinic acid, oxalic acid, malonic acid,tartaric acid, malic acid, glutaric acid, citric acid, maleic acid, andglycine.

The following examples are intended only to illustrate the invention andshould not be construed as imposing limitations upon the claims. Thefollowing experimental methods, conditions and instruments were employedin preparing the exemplary CMP particles detailed hereinbelow.

EXAMPLE 1

An abrasive particle suitable for CMP was prepared by first dissolving1854 g (3.38 mol) of Ce(NH₄)₂(NO₃)₆ in 3500 g of deionized water. Tothis solution, 100 g of Ti(isopropoxide)4 (0.35 mol) and 90 g of acetylacetone was added. The cerium salt solution was mixed with a basicsolution (containing 1000 g H₂O and 1043 g of KOH) using a controlleddouble jet injection method with constant stirring. The slurry thusformed was diluted to a volume of 8330 mL with deionized water, andheated in a sealed stainless steel vessel, i.e., a hydrothermal reactor,at 300° C. for 6 hours with agitation. After the slurry was dischargedfrom the hydrothermal reactor, the slurry was decanted several timesbefore being subjected to cross-flow washing until a conductivity of0.075 mS was achieved. The slurry was then subjected to a Dual-FrequencyReactor (Advanced Sonic Processing Systems) for sonification at a flowrate of about 50 ml/minute. The resultant final CeO₂ particles exhibiteda primary particle size of about 17 nm and a secondary particle size ofabout 800 nm. A slurry of 1 wt % CeO₂ particles was prepared in wateradjusted to a pH of 4 with HNO₃). This slurry was used for copper CMP ona Strasbaugh 6EC polisher with 3/1 psi & 60/60 rpm and with a slurryflow rate of 170 ml/min. A Cu removal rate of 1004 Å/min was obtained.The particle distribution was (d10) 50 nm, (d50) 120 nm, and (d90) 400nm, and the particles exhibited a crystallite size of 17 nm.

EXAMPLE 2

In a 500 ml beaker, 85.16 g of cerium ammonium nitrate Ce(NH₄)₂(NO₃)₆was dissolved in 300 ml deionized water, and 6.34 grams (0.0157 mol) offerric nitrate nonahydrate (Fe(NO₃)₃.9H₂O) was added with stirring. Thesolution was diluted further by addition of 350 ml of deionized water. Abasic solution was formed by dissolving 48.72 grams of potassiumhydroxide (KOH) in 350 ml deionized water. The solutions (1) containingcerium ammonium nitrate and ferric nitrate and (2) the base weresimultaneously introduced under stirring into a 1000 ml beaker via acontrolled double jet injection method. After the solutions were addedtogether, the resulting solution was stirred for 5 minutes and thenultrasonicated for 15 min. The solution was transferred to a sealed 1000ml stainless steel reaction cylinder, and then placed in a preheatedoven at 300° C. for 4 hours. The reaction cylinder was removed from theoven, cooled to room temperature, and the contents were transferred to a1000 mL plastic container. The reaction product consisted of adispersion of cerium oxide-ferric oxide composite nanoparticles. Theproduct slurry was then washed to remove excess unreacted ionic salts.The dispersion was ultrasonicated for 15 minutes and then filtered toafford a final product with pH 4±0.5 and conductivity less than 0.07 mS.The final product was filtered through 12-micron filter to removepossible external impurities. The Cu CMP slurry was made by diluting theabove particles to a 1 wt % dispersion using deionized water adjusted toa pH of 4 using nitric acid (HNO₃). The Cu CMP operation of Example 1was performed using the composite Ce—Fe particles this formed. A Curemoval rate of 1762 Å/min was obtained. This indicates that the Ce—Fecomposite particle is chemically more active than the pure CeO₂nanoparticles used in the comparative example. The particle distributionis (d10) 88 nm, (d50) 143 nm, and (d90) 346 nm, and the particlesexhibited a crystallite size of 10 nm.

EXAMPLE 3

A dispersion of cerium oxide-copper oxide composite nanoparticles wasformed using the same materials and procedure set forth in Example 1,except that 13.36 g of copper nitrate hemipentahydrate (Cu(NO₃)₂.2.5H₂O)was used instead of ferric nitrate nonahydrate. The Cu CMP was performedunder the same conditions as in Example 1. A Cu removal rate of 2021Å/min was obtained, indicating that Ce—Cu composite nanoparticles arechemically more active than pure CeO₂ particles. The particledistribution is (d10) 158 nm, (d50) 316 nm, and (d90) 747 nm, with acrystallite size of 7.4 nm.

EXAMPLE 4

A dispersion of cerium oxide-neodymium oxide composite nanoparticledispersion was formed using the same materials and procedure set forthin Example 1, except that 11.85 grams of neodymium (III) nitratehexahydrate (Nd(NO₃)₃.6H₂O) was used instead of ferric nitratenonahydrate. The Cu CMP was performed under the same conditions as inExample 1. A Cu removal rate of 1980 Å/min was obtained, indicating thatCe—Nd composite nanoparticles are chemically more active than pure CeO2nanoparticles. The particle distribution is (d10) 89 nm, (d50) 157 nm,and (d90) 437 nm and the particles exhibited a crystallite size of 6.3nm.

EXAMPLE 5

Composite cerium oxide particles were made in the following manner.Except for the identity and amount of guest reagent added, the syntheseswere substantially identical. In a 1000 ml beaker, 80.2 g (0.146 moles)of cerium ammonium nitrate Ce(NH₄)₂(NO₃)₆ was dissolved in 200 mldeionized water to form a well mixed aqueous solution of cerium salt. Tothe solution 18.95 g (0.036 moles) of acetyl acetone titanate was added.The salt solution was well mixed until it was clear and homogeneous. Asufficient quantity of DI-water was then added to reach a final volumeof 300 ml. In another 1000 ml plastic bottle, 48.7 g (0.338 moles) ofpotassium hydroxide (KOH) was added to DI water and diluted to a finalvolume of 300 ml. Using a controlled double jet injection the saltsolution and the base solution are mixed together with continuousagitation, and mixed an additional 5 minutes. The slurry is thenultrasonicated for 15 minutes. The slurry is then transferred to astainless steel reaction vessel and placed in a preheated oven at 300°C. for 4 hours. The stainless steel reaction vessel is removed from thefurnace and allowed to cool to room temperature. The reaction product (adispersion of cerium oxide-titanium oxide particles) is then transferredto a clean 1000 ml container. The dispersion was washed with DI waterseveral times to remove excess ions and to achieve subsequent separationfrom supernatants by settling. This was carried out several times untilthe ion concentration was very low (conductivity of 0.07 mS). The finalproduct was filtered through a 12-micron filter to remove any possibleexternal impurities.

FIG. 1 is a Table showing the molar amounts of guest ion reagents thatwere mixed with Ce(NH₄)₂(NO₃)₆ to produce the composite particles inaccordance with the invention. Sample 1 is the particle made by theexemplary synthesis of Example 5. The table further displays severalproperties of the composite particles. The secondary particle sizedistribution of the composite abrasive particles was determined by alight scattering technique as known in the art, using a Horiba LA-910particle size analyzer. The secondary particle size is defined as thesize of agglomerates of primary particles.

The slurry was adjusted to a pH of 4 and the solid content adjusted to1% before use in CMP. The particles have the polishing rate in angstromsper minute as set forth in FIG. 1. Polishing experiments were performedusing Westech 372 polisher using six-inch silicon oxide wafers. AnIC-1000/Suba-IV polishing pad was used for polishing. The downwardpressure on the carrier was set at 3.5 psi. The back pressure on thewafer carrier was 0 psi. The carrier was rotated at speed of 93 rpmwhile the pad was rotated at 87 rpm. The slurry feed rate to thewafer-pad area was 150 ml. The slurries used for the study aremaintained at room temperature (25° C.). The films studied were PECVDTEOS silicon dioxide on 150 mm diameter silicon wafer. The filmthickness, before and after CMP, was measured optically using aPrometrics FT-750.

It is noted that sample 34 in Table 1 is a comparative example, inasmuchas it is a pure cerium oxide particle. Lacking guest metal ions, thecerium oxide particle of sample 34 exhibits a polishing rate on asilicon dioxide substrate of 3.14 Å/min.

EXAMPLE 6

To study the STI polishing performance of abrasive particles withdifferent mole percentages of titanium ions in the crystal structure, 11samples of abrasive particles were synthesized using the procedures andreagents described in Example 5 above to produce particles having themole percentages of ceria and/or titania shown in Table 1 below: TABLE 1Sample Ce/Ti (mole %) Raw D₅₀ Raw Dmean (nm) 6-1 100/0  78 426 6-2 90/10473 504 6-3 80/20 249 247 6-4 70/30 561 677 6-5 60/40 714 719 6-6 50/50918 918 6-7 40/60 799 803 6-8 30/70 852 854 6-9 20/80 67 79  6-10 10/9067 70  6-11  0/100 69 71

The particles were dispersed in water at a weight percent loading of 1%.HNO₃ was added to adjust the pH of the slurries to 4.0. No chemicaloxidizers and/or surfactants were added to the slurries, and theslurries were not sonicated or filtered after formation.

Samples 6-1 through 6-11 were used separately to polish 6-inch blanketthermal oxide (TOX) and nitride (NIT) wafers using a Westech 372polisher, an IC-1000/Suba-IV pad, with a downward pressure of 3.5 psi(no back pressure), a carrier/pad rotation speed of 93/87 rpm, and aslurry feed rate of 150 ml/min. Polishing was conducted using theslurries first on a dummy silicon wafer, then on a used TOX wafer, thenon a new TOX wafer and finally on a new NIT wafer. Distilled water wasused to clean the wafers, which were air-dried. Polishing results areshown in Table 2 below, where “WIWNU” means WIthin Wafer Non-Uniformity:TABLE 2 Polish rate (nm/min) Polish rate (nm/min) Polish rate (nm/min)Surface TOX/NIT Sample TOX Used WIWNU TOX2 New WIWNU Avg TOX NIT WIWNURough (nm) Selectivity 6-1 4.1 35.53% −1.2 37.60% 1.45 0.5 47.59%  1-1.5 2.9 6-2 339.1 24.04% 261.9 26.48% 300.5 197.9 6.38%   1-1.21.518444 6-3 43.6 34.53% 36.3 36.04% 39.95 224.3 7.09% 1.1-1.5 0.178116-4 399.5 15.45% 232.5 15.80% 316 234.6 5.10% 0.9-1.5 1.346974 6-5 335.911.74% 352.5 9.72% 344.2 159.2 5.05% 1.7-1.9 2.16206 6-6 300.1 12.32%260.4 13.50% 280.25 18.3 36.33% 0.8-2.0 15.31421 6-7 196.4 16.85% 168.316.74% 182.35 2.7 33.11% 2.1-3   67.53704 6-8 260.6 9.40% 251.9 11.98%256.25 96.3 6.50% 1.1-1.4 2.660955 6-9 90.9 16.29% 110.8 17.83% 100.852.7 37.34% 1.4-1.9 37.35185  6-10 N/A N/A 4.6 29.61% 4.6 28.9 22.32%1.2-1.5 0.15917  6-11 N/A N/A 19.7 29.09% 19.7 23.3 26.34% 1.1-1.20.845494

The results shown in Table 2 above show that pure ceria particles (i.e.,containing no titanium ions) did not polish TOX or NIT wafers at a veryhigh rate. The removal rate increased as titanium ions were introducedinto the ceria crystal structure, and then decreased as theconcentration of titanium ions reached about 70 mole percent. In thecase of pure titania, both TOX and NIT removal rates were very low dueto the inherent properties of titania particles.

The TOX wafer surface for the slurries containing between 50 mole % and80 mole % titanium ions are very rough due to the large secondaryparticle sizes and the presence of needle-like particles, which lead tosome scratches on the wafers. In the case of 70 mole % titanium ions, abetter result may be attributable to a comparatively small afterformulation secondary particle size (Dmean=338 nm vs. 404-1188 nm). Inthe range of 10 mole % to 40 mole % titanium ions, the TOX polishingrate is high and the nitride removal rate is ideally not very high. Theresults for Sample 6-3 (i.e., the 20 mole % titanium ions sample) arequestionable and may not be reliable.

The primary particle size increased in relation to the increases intitanium ions present, with the lattice constant decreasing from 5.42 Åfor pure ceria to 3.81 Å for pure titania. The lattice constant remainedsimilar to pure ceria until the mole percentage of titanium ions reachedabout 50 mole %. After that point, the crystal structure of theceria-titania composite abrasive particles became more similar titania,which means that cerium ions were guest ions and titanium ions were thehost ions in the crystal structure, which was anatase. Particles of thistype were not as good for oxide polishing as particles that exhibited asubstantially cubic crystal structure.

EXAMPLE 7

Abrasive particles were formed in accordance with the procedures andusing the reagents and equipment as described in Example 5, except thatthe mole percentage of titanium cations to cerium cations was 5.95% (Ti)to 94.05% (Ce). The raw mean particle size (D₅₀) of the abrasiveparticles was 79 nm. The abrasive particles were dispersed at differentloadings in water to form slurries. No chemical oxidizers or surfactantswere added, and the slurries were not sonicated or filtered. The pH ofthe slurries was adjusted using HNO₃ or KOH. Slurry 7-1 included 1.0% byweight of the abrasive particles and had a pH of 4. Slurry 7-2 included1.0% by weight of the abrasive particles and had a pH of 10. Slurry 7-3included 0.5% by weight of the abrasive particles and had a pH of 4.And, Slurry 7-4 included 1.5% by weight of the abrasive particles andhad a pH of 4.

The slurries were separately used to 6-inch blanket thermal oxide (TOX)and nitride (NIT) wafers using the same equipment and procedures used inExample 6. The polishing results are shown in Table 3 below: TABLE 3Polish rate (nm/min) Polish rate (nm/min) Polish rate (nm/min) SurfaceTOX/NIT Sample TOX Used WIWNU TOX2 New WIWNU Avg TOX NIT WIWNU Rough(nm) Selectivity 7-1 15.9 28.18% 19.1 9.29% 17.5 148.45 1.73% 1.1-1.30.1178848 7-2 117.6 39.52% 92.6 11.78% 105.1 124.4 23.37% 1.3-1.40.8448553 7-3 179.6 11.95% 152.2 22.72% 165.9 114.6 20.37% 1.2-2.11.447644 7-4 37.7 22.79% 24.4 14.65% 31.05 197.75 34.92% 0.9-1.00.1570164

The basic pH value 1 0 may be a little closer to the IEP ofhydrothermally created ceria particles (IEP=˜8.5), resulting in apronounced increase of secondary particle size after slurry formulation(Dmean=416 nm at pH=10 vs. 149 nm at pH=4), and thus higher oxideremoval rate and rougher surface finish. It is somewhat surprising thatan increase in TOX polishing rate was observed when the weight percentof abrasive particles at a pH of 4 was reduced from 1.0% (Slurry 7-1) to0.5% (Slurry 7-3). It is possible that the relatively large secondaryparticles from Slurry 7-2 may have contaminated and been retained asresidue on the polishing pad during the polishing with Slurry 7-3. It isalso possible that the more dilute slurry (Slurry 7-3) provides moreopportunities for the abrasive particles to contact the wafer and padand thus results in a higher removal rate.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and illustrative examples shown anddescribed herein. Accordingly, various modifications may be made withoutdeparting from the spirit or scope of the general inventive concept asdefined by the appended claims and their equivalents.

1. A method of producing abrasive particles for use in CMP slurriescomprising: a. providing an aqueous reaction mixture comprising i. oneor more compounds that provide a source of cerium ions, ii. one or morecompounds that provide a source of metal ions selected from the groupconsisting of Be, B, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn,Ga, Ge, As, Sr, Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te,Ba, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Hp, Er, Tm, Yb, Lu, Hf, Ta, W,Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, and combinations thereof, b.contacting the aqueous reaction mixture with a base to raise the pH toabove about 1.5, and, c. subjecting the aqueous reaction mixture tohydrothermal treatment at a temperature of from about 70° C. to about500° C. to produce the abrasive particles, d. wherein the abrasiveparticles comprise crystallites having crystal lattice structures thatinclude cerium and one or more metals other than cerium.
 2. The methodof claim 1 wherein the ratio of cerium ions to guest ions is about100000:1 to about 1:100000.
 3. The method of claim 1 where the compoundthat provides the source of cerium ions is a Ce(III) salt or a Ce(IV)salt.
 4. The method of claim 1 wherein the reaction mixture is subjectedto hydrothermal treatment for about 10 minutes to about 48 hours.
 5. Themethod of claim 4 wherein the aqueous reaction mixture is contacted withthe base by double jet injection.
 6. The method of claim 5 wherein thecompound that provides the source of guest ions is a salt of the guestion.
 7. The method of claim 6 wherein the compound that provides thesource of guest ions is selected from the group consisting of Fe(NO₃)₃,Cu(NO₃)₂, Nd(NO₃)₃, and hydrated forms thereof.
 8. The method of claim 1wherein the particles have a crystallite size of about 5 to about 100nm.
 9. The method of claim 1 wherein the particles agglomerate to form asecondary particle size of about 50 to about 500 nm.
 10. A method ofmaking composite CMP ceria particles comprising: a. contacting anaqueous solution of Ce(NH₄)₂(NO₃)₆ with a second metal salt to form areaction mixture; b. contacting the reaction mixture with a base toraise the pH to above about 1.5; and c. heating the reaction mixture toform the particles.
 11. The method of claim 10 wherein the secondreaction mixture is heated for about 10 minutes to about 48 hours at atemperature of about 70° C. to about 500° C.
 12. The method of claim 10wherein the first reaction mixture is contacted with the base by doublejet injection
 13. The method of claim 10 wherein the second metal saltis selected from the group consisting of nitrates, chlorides, bromides,sulfates, perchlorides, and acetates of iron, copper and neodymium intheir anhydrous and hydrated forms.
 14. The method of claim 10 whereinthe second metal salt is selected from the group consisting of Fe(NO₃)₃,Cu(NO₃)₂, Nd(NO₃)₃, and hydrated forms thereof.
 15. A method of removinga film at a desired rate in the absence of chemical oxidizerscomprising: a. determining the desired polishing rate of film to beremoved; b. selecting abrasive particles according to claim 1 thatprovide a desired polishing rate for the film be removed; and c.polishing the film with a CMP slurry comprising the particles selectedin step b.
 16. The method of claim 15 wherein the film be removed isselected from the group consisting of silver, gold, platinum, copper,palladium, nickel, cobalt, iron, ruthenium, iridium, and osmium,silicon, aluminum, germanium, tungsten, tantalum, and alloys or blendsthereof.
 17. The method of claim 15 wherein the film to be removed isselected from the group consisting of oxides, nitrides or silicides ofboron, sodium, magnesium, aluminum, silicon, phosphorus, potassium,calcium, gallium, germanium, arsenic, selenium, rubidium, strontium,yttrium, zirconium, tin, antimony, cesium, nickel, cobalt and barium.18. The method of claim 15 wherein the film to be removed is a polymeris selected from the group consisting of poly(para-xylylenes),halogenated poly(para-xylylenes), b-staged polymers, polyimides,halogenated polyimides, silsequioxanes, alkyl substitutedsilsequioxanes, poly-(arylene ethers) and poly-(tetrafluoroethylene).19. The method of claim 15 wherein the CMP slurry further comprises a pHadjuster.
 20. A CMP slurry comprising: a. water; and b. abrasiveparticles according to claim
 1. 21. The CMP slurry of claim 20 whereinthe slurry is substantially free of chemical additives/oxidizers. 22.The CMP slurry of claim 20 wherein the ratio of cerium ions to guestions is about 1000:1 to about 1:1000.
 23. The CMP slurry of claim 21wherein the guest ion is selected from the group consisting of Fe, Nd,and Cu.
 24. The CMP slurry of claim 21 wherein the guest ion is selectedfrom the group consisting of Ti, Ta and Y.
 25. A method of removing aportion of a substrate in a CMP operation comprising: a. providing theCMP slurry of claim 20; b. adjusting the pH of the slurry to 3.0 to 11.0using at least one pH adjuster; c. contacting the slurry and thesubstrate to be polished; and d. performing CMP on the substrate usingsaid slurry.
 26. The method of claim 25 wherein the difference inelectronegativity of the cerium ions and the guest ions is sufficient todrive a redox reaction between the particle and the substrate when theparticle contacts the substrate.